Microdialysis probe technology provides a means for sampling diffusible tissue extracellular constituents. However, the relationship of the probe effluent concentrations to their tissue counterparts is a complex function of solute molecular weight, solute physicochemical properties, tissue properties, probe membrane properties, probe geometry and perfusion rate, and the trauma of probe insertion into the tissue. These dependencies will be studied in order to improve the quantitative usefulness of the technology. The focus of applications is the brain in connection with Parkinson's disease and transplant therapy. Solutes of particular interest are dopamine and its metabolites. Approaches considered include mathematical modeling of solute transport within the probe and surrounding medium with both in vitro and in vivo validation experiments.